A hollow metallic head has been widely used as a wood type golf club head such as a driver and a fairway wood. In general, a hollow wood type golf club head includes a face part, a crown part, a sole part, a side part and a hosel part. The face part is for hitting a ball. The crown part comprises an upper surface of the golf club head. The sole part comprises a bottom surface of the golf club head. The side part comprises lateral surfaces of the head at the toe and back and heel sides. A shaft is inserted to the hosel part, fixed with adhesive. Recently, a number of new golf clubs, called utility clubs, have been sold on the market. Various types of golf clubs having hollow heads (namely, heads having face part, sole part, side part, crown part and hosel part), which resemble the above wood type golf club head, also are sold as one type of these utility golf clubs.
For the metals configuring the hollow golf club heads, aluminum alloys, stainless and titanium alloys are used. Recently, the titanium alloys are widely used.
Patent Document 1 describes that a launch angle is enlarged as well as resilience is improved by providing a slit on a crown part of a hollow golf club head. Patent Document 1 further describes that the slit is covered/filled with a reinforce material that is made of material whose elasticity is lower than that of a metal.
Patent Document 2 describes that, in order to decrease a metal sound at hitting with a hollow golf club head, a penetration hole with 5 mm to 15 mm diameter is provided on its crown and a sound deadening material made of polyester elastomer, thermoplastics resin, or rubber is fit/fixed in the penetration hole.
Patent Document 3 describes that, in a golf club head in a shell structure, a penetration groove or penetration hole having more than 20 mm length in a longitudinal direction of a topline is provided on a crown part and the penetration groove or penetration hole is filled with a material having Young modules smaller than other portions.
Patent Document 4 describes that a stress reducing structure is provided on a crown part and sole part of a golf club head having a hollow part.
Patent Document 5 describes a structure in which an opening is provided on a top part of body of a hollow golf club head and the opening is closed with a fiber content hybrid material such as carbon filling nylon.
Patent Document 6 describes that a slot is provided on a crown part and an elastomer filling material is provided in the slot.